Two physical effects are mainly used to detect the presence of smoke, namely the scattering of light by the smoke, dust or aerosols associated with it; and the change in the movement speed of ions driven by an electric field as a result of this smoke, dust or aerosols.
The devices that exploit the second effect by using air ionization are more sensitive to combustion products, emitted during the initial development of fires or in hot fires; the size of these products can reach values of several tens of nm, or less, and thus allow alarms to be triggered earlier than optical devices. As a result, these detectors make it possible to limit the consequences of these fires.
An ionic smoke detector comprises a chamber in which two measurement electrodes are arranged between which qi charged ions are created or brought.
Applying a potential difference between these electrodes produces an electric field E that exerts a force F=qi×E on these ions, which produces a nominal electric current between the electrodes and in the external circuit that connects them. This electric current is dependent in particular on the quantity of ions present in the chamber, the potential difference applied between the measurement electrodes, and the mobility of the ions.
Means of measuring this current are also provided, which supply a signal that can be used by processing means.
When particles associated with smoke enter the chamber, some of these particles attach to the chamber's ions as a result of the electrostatic forces created by these ions, which reduces their mobility and has the effect of reducing the electric current.
If the voltage applied to the electrodes is low enough, typically between 5 and 30 volts, the nominal electric current is also low, typically between 10 pA and 100 micro-amperes, and the slowing down of the ions resulting from the presence of the particles is such as to reduce the amplitude of this current very substantially.
The processing means are arranged so as to allow an alarm to be triggered or sent when the current measured is below a predefined threshold.
Two approaches have been used to create ions in the measurement chamber, either by ionizing the air using a small radioactive source, as described, for example, in patent FR 86 02567, or by creating an electric field stronger than the electric field for air breakdown, as described, for example, in U.S. Pat. No. 3,823,372.
The first approach is simple to implement and not very costly.
For example, a source of α particles comprised of Am-241 with activity between 0.1 and 1 microcurie is used, these particles being able to cross a distance of the order of centimeters in the air and thus ionize the volume passed through.
However, although this solution makes it possible to detect fires early and thus reduce their consequences, it is facing increasing challenges, either from users themselves, who are reluctant to find increased numbers of radioactive sources in their premises, or from manufacturers' sales departments, who are confronted by negative reactions from their customers, or from regulations.
With regards to the second approach, various solutions have been proposed for ionizing the air.
They use the fact that, by applying a potential difference above a certain threshold Vs between two electrodes, one can initiate a process of electrical discharge and thus create ions.
The value Vs depends on several parameters, such as the nature of the gas between the electrodes, the pressure of the gas separating them, the distance between the electrodes and their shape, the presence of dust or humidity, etc.
In the air, this threshold is considered to be approximately 330V for distances between electrodes of the order of micrometers, distances too small to be used directly in a smoke detector, which means that voltages of several kilovolts must be used to create this ionization.
However, values such as these cannot be used for polarizing the measurement electrodes since the high speed of the ions resulting from this would lead to a very high nominal electric current and these ions would cross the measurement chamber in a very short period of time.
As a result, the changes in this current because of the presence of particles associated with smoke would be so small that they would be difficult to detect.
To overcome this obstacle, various proposals have been made using a measurement chamber polarized by a weak voltage, and thus having a low nominal current.
A first approach has been to use a measurement chamber polarized by a weak voltage, into which ions produced in an ionization chamber polarized by a high voltage are transferred by means of a weak current of air, and thus to have a low nominal current.
An example of such a solution is described in patent FR 96 03296.
In a second approach, reflective elements have been introduced between the electrodes of an ionization chamber polarized by a high voltage, so as to increase the interaction time.
An example of such a solution is described in U.S. Pat. No. 3,932,851.
These alternative solutions, however, result either in detectors that have relatively low detection sensitivities due to the very fact of using high voltages, significantly reducing their advantages, or in devices that are mechanically complex, fragile and expensive.
In addition, the response of these detectors is also influenced by parameters such as variations in ambient gas pressure or in temperature, thus requiring compensation devices to be used as well, such as described in patent EP-236223, for example.
For these reasons there have been no major industrial-scale developments of these alternative solutions.